(1) Field of the Invention
This invention relates to a planographic printing plate which is suitable for use in direct printing on a material to be printed.
(2) Description of the Prior Art
Printing plates may be classified into two categories, that is, those (first category) which are suitable for use in direct printing wherein an image is transferred from the printing plates directly onto the material to be printed, and those (second category) which are suitable for use in indirect printing wherein an image is transferred from the printing plates through an intermediate carrier onto the material to be printed. The indirect printing of the second category is usually called offset printing. The direct printing of the first category includes typically letterpress printing and gravure printing. However, offset printing can be effected by using letterpress printing plates and direct printing can be effected by using lithographic printing plates.
Newspaper printing has heretofore been conducted mainly by letterpress printing. However, lithographic offset printing and lithographic direct printing are now attracting people's attention for newspaper printing.
Conventional lithographic printing plates have some problems when they are used for direct printing. This is because dampening water is used in order to maintain the non-image area ink-repellent. For example, in the case where paper is used as the material to be printed, a relatively large amount of dampening water is absorbed by the paper when the paper is brought into direct contact with the printing surface of the printing plate. Consequently, the proportion of the dampening water to the printing ink becomes unbalanced, and the paper is subject to dimensional change and the rolled paper is liable to break. Even where the paper used is non-water-absorptive, the dampening water on the printing plate causes some problems. In the direct printing process, a larger amount of the ink on the image area of the printing plate is transferred onto the material to be printed as compared with the offset printing process. However, in conventional lithographic printing plates, it is difficult to maintain the proportion of dampening water to printing ink at the desired value. Furthermore, it is difficult to control the thickness of the layer of printing ink on the printed material or to increase the thickness of the ink layer to a depth such as in screen printing or gravure printing.
In planographic offset printing, a rubber blanket, which receives printing ink from the printing plate and transfers it onto the material to be printed, ensures uniform printing. In contrast, in direct planographic printing wherein a printing ink is directly transferred from the printing plate onto the material to be printed, it is difficult to apply a completely uniform printing pressure over the entire material to be printed. When the material to be printed has a rough surface including fine convexities and concavities such as those formed by fibers of paper, the ink is preferentially transferred onto the convexities present on the surface of the material to be printed. As means for eliminating this defect, Japanese Patent Publication No. 9323/76 and U.S. Pat. Nos. 4,112,841 and No. 4,114,535 proposed planographic printing plates for direct printing, which comprise a printing layer formed on a rubber elastic layer. In these planographic printing plates, the rubber layer formed below the printing layer acts as a blanket used in offset printing, and this rubber layer ensures good transfer of an ink. However, dampening water must be used and, therefore, these printing plates still have the above-mentioned defects of the conventional dampening planographic printing plates. Furthermore, these printing plates cannot accept a large amount of a printing ink thereon.